1. Field of the Invention
The present invention relates to a combustion apparatus for burning a liquid fuel.
2. Description of Related Art
Some combustion apparatuses known in the art are of the type as disclosed in Patent Laying-Open Gazette No. 10-227453. A fuel spraying nozzle incorporated in this apparatus operates to blow a fuel mist to be burnt continuously. This nozzle is of the so-called return type that has an internal return path such that a portion of the fuel supplied from a fuel tank will flow back toward the tank through the internal return path and a return channel provided out of the nozzle.
FIG. 18 is a flow diagram showing the flow of fuel in the related art combustion apparatus that includes the return type nozzle. A fuel spraying nozzle 205 built in this apparatus 201 has a spray mouth for jetting a fuel mist. A fuel channel (or “a fuel canal”) 209 connected to the nozzle 205 is composed of a feed channel (or “a feed canal”) 210 reaching the spray mouth and a return channel (or “a return canal”) 211 leading back therefrom to an upstream region of said channel. The feed channel 210 starting from a fuel tank 214 so as to terminate at an inlet of the spraying nozzle 205 does include electromagnetic valves 212 and 213 and an electromagnetic pump 215 that are arranged in series along the feed channel. On the other hand, the return channel 211 connected to a returning side of the nozzle 205 does include a check valve 216 and a proportional control valve 217, that are likewise arranged in series. A downstream end of the return channel 211 merges into the feed channel 210, at a junction intervening between the electromagnetic valve 212 and the electromagnetic pump 215.
The proportional control valve 217 disposed in the return channel 211 is the so-called “ball type” valve that cannot absolutely close this channel 211. Therefore, the one electromagnetic valve 212 is interposed between the junction and the fuel tank 214 so as to avoid any excessive or undesired flow of fuel from or towards this tank.
FIG. 19 shows the structure of proportional control valve 217 employed in the related art combustion apparatus 201. This valve has an internal fuel passage 221 formed in a casing 220 and extending between a fuel inlet end 222 and a fuel outlet end 223, with the inlet end 222 leading to the check valve 216. A valve seat 225 is formed at an intermediate point in the internal passage 221, and a spherical valve body 226 rests on this seat 225. A plunger 227 in contact with the valve body 226 is surrounded by an electromagnetic coil 228. With this coil being turned on with an electric current, it will make a stroke along the axis of casing so as to move the valve body 226 up and down.
As the plunger 227 displaces the valve body 226, the cross-sectional area of internal passage 221 will vary to change the flow rate of fuel advancing from the inlet end 222 to outlet end 223. A current regulator not shown but varying the intensity of electric power applied to the proportional control valve 217 will serve to control the fuel flow rate through the return channel.
The fuel stream effluent from the tank 214 will continuously be compressed in the electromagnetic pump 215, before entering the spraying nozzle 205.
The thus compressed fuel stream of a high pressure will reach the spray mouth that is located at a distal end of the spraying nozzle 205, so that a noticeable portion of such a fuel stream is blown outwards to form a mist. The remainder of said fuel stream will flow back from this nozzle 205, through the check valve 216 and into the inlet end 222 of proportional control valve 217. The remainder having entered this valve 217 through its inlet end 222 is delivered to an upstream region of the feed channel, at a flow rate determined by the intensity of current being applied to said coil 228.
Gradual change or certain fluctuation in the temperature of the proportional control valve 217 has been observed in the related art combustion apparatus 201 during its operation. Such a change or fluctuation as being caused by the change in ambient temperature and/or the like will in turn change the temperature of coil 228 installed in the casing 220. Electric resistance of the coil 228 will vary in response to the change in its temperature, thereby rendering unstable the current intensity applied to the coil 228. Consequently, the flow rate at which the remainder of fuel stream flows back through the return channel will become unreliable. It has been somewhat difficult for the related art apparatus 201 to precisely regulate the spraying rate of fuel, failing to stabilize the condition of combustion state.
Such an unstable combustion in the related art apparatus does mean that the amount of a fuel sprayed out of said nozzle would not be burnt completely. Incomplete combustion will result in the discharge of a non-burnt fraction, bringing about a poorer efficiency of energy. In addition, an unnegligible amount of toxic gasses such as carbon monoxide is likely to be discharged to the outside, and an undesirable accumulation of soot will take place inside the apparatus. Thus, the problem of environmental pollution has been inherent in the related art combustion apparatuses, not only rendering them likely to become out of order.